Conventional security equipment tends to be expensive and is generally not suited for protecting valuables carried in transportable containers. The contents of small cash boxes such as those commonly carried in taxis, and the often very valuable contents of mail bags are particularly vulnerable to theft, frequently under threat of arms or violence. The present invention enables protection to be provided in these circumstances quite cheaply but most effectively. The invention does, however, have application to large scale security installations because it enables the provision of security apparatus which is virtually tamperproof and cannot be rendered ineffective as in the case of even quite sophisticated conventional electrical systems.